Abstract

N-4 is defined to be the fraction of borons in a borate which are four-coordinated, and this can be determined reliably by NMR measurements. Although the composition dependence of N-4 has been measured for numerous glass forming systems, little work has been done to understand its behaviour. We have used the most extensive set of N-4 data in the literature [Jellison Jr., G. E., Feller, S. A. & Bray, P. J. Phys. Chem. Glasses, 1978, 19, 52], for lithium borate glasses, to develop a parameterisation for the composition dependence of N-4. This involves a conversion factor, a, which has the maximum value of two for low modifier content, but decays exponentially above a limiting composition, which occurs when there is a network of alternating BO3 and BO4 units. Above this composition the creation of BO4 units is not favoured because this brings centres of negative charge into close proximity. Our parameterisation is empirical, but has potential for use in the characterisation and comparison of N-4 data from a wide variety of borate glass systems.